The present invention relates to an implantable microphone system useable with cochlear implants or implantable hearing aids, and more particularly to an implantable microphone system that uses a highly sensitive motion/position sensor that may be coupled to middle ear structure.
A cochlear implant is an electronic device designed to provide useful hearing and improved communication ability to individuals who are profoundly hearing impaired and unable to achieve speech understanding with hearing aids. Hearing aids (and other types of assistive listening devices) make sounds louder and deliver the amplified sounds to the ear. For individuals with a profound hearing loss, even the most powerful hearing aids may provide little to no benefit.
A profoundly deaf ear is typically one in which the sensory receptors of the inner ear, called hair cells, are damaged or diminished. Making sounds louder or increasing the level of amplification, e.g., through the use of a hearing aid, does not enable such an ear to process sound. In contrast, cochlear implants bypass damaged hair cells and directly stimulate the hearing nerves with electrical current, allowing individuals who are profoundly or totally deaf to receive sound.
In order to better understand how a cochlear implant works, and how the present invention is able to function, it is helpful to have a basic understanding of how the ear normally processes sound. The ear is a remarkable mechanism that consists of three main parts: the outer ear, the middle ear and the inner ear. The outer ear comprises the visible outer portion of the ear and the ear canal. The middle ear includes the eardrum and three tiny bones. The inner ear comprises the fluid-filled snail-shaped cochlea, which contains thousands of tiny hair cells.
When the ear is functioning normally, sound waves travel through the air to the outer ear, which collects the sound and directs it through the ear canal to the middle ear. The sound waves strike the eardrum (tympanic membrane) and cause it to vibrate. This vibration creates a chain reaction in the three tiny bones in the middle ear. These three tiny bones are medically termed the malleus, incus and stapes, but are also commonly referred to as the xe2x80x9chammerxe2x80x9d, xe2x80x9canvilxe2x80x9d and xe2x80x9cstirrupxe2x80x9d. Motion of these bones, in turn, generates movement of the oval window, which in turn causes movement of the fluid contained in the cochlea.
As indicated above, the cochlea is lined with thousands of tiny sensory receptors commonly referred to as hair cells. As the fluid in the cochlea begins to move, the hair cells convert these mechanical vibrations into electrical impulses and send these signals to the hearing nerves. The electrical energy generated in the hearing nerves is sent to the brain and interpreted as xe2x80x9csoundxe2x80x9d.
In individuals with a profound hearing loss, the hair cells are damaged or depleted. In these cases, electrical impulses cannot be generated normally. Without these electrical impulses, the hearing nerves cannot carry messages to the brain, and even the loudest of sounds may not be heard.
Although the hair cells in the cochlea may be damaged, there are usually some surviving hearing nerve fibers. A cochlear implant works by bypassing the damaged hair cells and stimulating the surviving hearing nerve fibers, or ganglion cells, with an electrical signal. The stimulated nerve fibers then carry the electrical signals to the brain, where they are interpreted as sound.
Representative cochlear implant devices are described in U.S. Pat. Nos. 4,267,410; 4,428,377; 4,532,930; and 5,603,726, incorporated herein by reference.
Cochlear implants currently use external microphones placed on the body that pick up sound (sense acoustic pressure waves and convert them to electrical signals) and then transmit the electrical signals to a signal processor for amplification, processing and conversion into an electrical stimulation signal (either current or voltage) that is applied to the surviving acoustic nerves located in the cochlea. Such a microphone is, by design, very sensitive, and in order to be sensitive, is by its nature very fragile. Disadvantageously, the external microphone can be damaged if it becomes wet, is dropped or is exposed to extreme conditions frequently encountered in the external environments. These fragile and sensitive microphones also restrict the user""s lifestyle and activities. For example, when a user must wear a microphone, he or she is typically restricted from participation in swimming and other sports, e.g., contact sports, unless the microphone is removed during such activities. If the microphone is removed, however, the user no longer is able to hear. Moreover, many users also find external microphone cosmetically objectionable since they appear out of place and mark the user as xe2x80x9cneeding assistancexe2x80x9d.
Middle ear microphones are known in the art. Disadvantageously, however, such prior art middle ear microphones typically require that sensors be attached between moving middle ear structures and stationary parts of the middle ear. This attachment may constrain motion and reduce or modify the performance of these moveable middle ear structures, resulting in an undesirable frequency response and/or distortion in sounds that are perceived. Further, adding too much mass to the ossicle chain or other moving structures of the middle ear may also change the dynamic behavior of the middle ear. What is needed, therefore, relative to a middle ear microphone, or a microphone coupled to middle ear structure, is a microphone that preserves the structure and dynamic performance of the middle ear as much as possible.
An example of an implantable microphone is found in U.S. Pat. No. 5,814,095, incorporated herein by reference. One technique for mounting such a microphone near the ear canal is shown in U.S. Pat. No. 5,999,632. When mounted as disclosed in the 5,999,632 patent, the implantable microphone disclosed in the U.S. Pat. No. 5,814,095 patent is not implanted in the middle ear, but is acoustically coupled to the outer ear.
From the above, it is thus evident that improvements are needed in the way users of a cochlear implant, or other hearing aid systems, sense or hear sounds, and more particularly, it is evident that improvements are needed in the microphones used with such systems, including implantable microphones coupled to middle ear structure.
The present invention addresses the above and other needs by replacing the external microphone commonly used with cochlear implants or other hearing aid systems with an implantable microphone system. Advantageously, such implantable microphone system detects xe2x80x9csoundxe2x80x9d by sensing the motion of middle ear components, e.g., by sensing the motion of the ossicles, without seriously degrading the performance of the middle ear components.
In one embodiment, a linear-variable-differential transformer (LVDT) is used within the middle ear to sense very small motion and position. A movable magnetic core of the LVDT is attached to the ossicies or other moving structure within the middle ear in order to measure their relative motion. Advantageously, the movable core need not be attached to any other structure. As the core of the LVDT is displaced from side-to-side by motion of the ossicles (or other movable members within the middle ear), which motion is created by audio signals (sound waves) that impinge upon the tympanic membrane, a modulated signal is induced in the windings of the transformer. This modulated signal has a phase change associated therewith that can readily be detected using conventional detection means. Such detected phase change may then be readily converted into a representation of the audio signal impinging upon the tympanic membrane.
In another embodiment, the movable plate of a movable-plate differential capacitor (MPDC) is attached to the ossicles or other movable structure of the middle ear in order to measure the relative motion thereof. Advantageously, the movable plate of the MPDC need not be attached to any other structure. As the plate of the MPDC is displaced from side-to-side by motion of the ossicles (or other movable members within the middle ear), which motion is created by audio signals (sound waves) that impinge upon the tympanic membrane, a modulated charge or voltage signal is created that is present on the non-moving plates of the MPDC. The modulation of this signal can readily be detected using conventional detection means. Such detected modulation may then be readily converted into a representation of the audio signal impinging upon the tympanic membrane.
In broad terms, the invention may be summarized as an implantable microphone system that includes: (1) a sensor having a movable member for sensing motion of moveable middle ear components, wherein the movable sensor member is adapted to be coupled only to a moving member within the middle ear, without being attached to any other middle ear elements, i.e., so that the movable member xe2x80x9cfloatsxe2x80x9d with the movable middle ear member, and wherein the sensor is implantable within the middle ear; and (2) circuit means coupled to the sensor for generating a modulated signal that varies as a function of the relative motion of the movable sensor member. Advantageously, the modulated signal thus functions as a microphone output signal that varies as a function of acoustic sound waves received through the outer ear and impressed upon the movable middle ear components.
In accordance with one aspect of the invention, the surviving tympanic membrane or other middle ear components is/are used as an effective diaphragm for a fully implanted microphone. Even though hearing may be lost, most individuals who are characterized as profoundly deaf still have a fully functioning tympanic membrane and middle ear components. The present invention advantageously relies on the response of such fully functioning tympanic membrane or other middle ear components as incoming acoustic pressure waves are received in the outer ear and funneled into the ear canal. The acoustically induced vibrations in any of these moving components in the middle ear is detected using, in one preferred embodiment, a LVDT transformer having a movable core that is coupled to a moving member of the middle ear, and in another preferred embodiment, a MPDC capacitor having a movable plate coupled to a moving member of the middle ear.
In operation, the the LVDT has three windings and a movable magnetic core. The movable magnetic core is adapted to be coupled only to a movable member of the middle ear, e.g., the ossicle chain. In one embodiment, the movable core may actually replace one or more of the tiny ossicle bones. The LVDT has three groups of windings, L1, L2 and L3, in addition to the movable magnetic core. These windings are not physically connected to the magnetic core, but are magnetically linked thereto. The movable magnetic core is attached to one of the moving structures of the middle ear so that it is positioned approximately equally between coils L1 and L2, which coils function as primary windings of the transformer. The signals impressed on coils L1 and L2 are magnetically coupled, through the movable core, to the third winding L3, which functions as a secondary winding of the transformer. The signal applied to coil L2 is set to be 180 degrees out of phase with the signal applied to coil L1, resulting in the signal induced in coil L3 nominally being zero. However, as the core is displaced from side-to-side because of the motion of the middle ear movable member, more (or less) coupling is provided from coil L1 to coil L3 than is provided from coil L2 to coil L3. The net result is that a modulated signal is provided at coil L3 that has a phase change that can be detected through conventional means and turned into a representation of the audio signal that impinges on the tympanic membrance.
Advantageously, by using the movable core of the LVDT as the moving element, the need for flexing wire connections is eliminated. Also, if desired, the number of connections can be reduced by inverting the connections on coil L2 and connecting them in parallel to the connection to coil L1.
For the embodiment where the MPDC is utilized, the movable plate is coupled or attached to a movable member of the middle ear, and the operation is similar. That is, the MPDC has a first plate P1, a second plate P2, an output plate P4 and a movable plate P3. The capacitance sensed at plate P4 is a function of the spacing between the movable plate P3 and the plates P1 and P2. By applying a signal to plate P2 that is 180 degrees out of phase relative to the signal applied to plate P1, the nominal output sensed at plate P4 is zero. However, as the position of the movable plate P3 varies, as sound impinges upon the tympanic membrane and causes the middle ear members to move, a signal is detectable at the output plate P4 that represents the audio signal impinging upon the tympanic membrane.
The movable core of the LVDT, or the movable plate of the MPDC, can be attached to one of the existing ossicle bones, or to the tympanic membrane or to the oval window. Alternatively, the movable core or movable plate may be built into a prosthetic type of device that is used in a standard stapedectomy procedure, i.e., a procedure where the prosthetic device replaces one or more of the ossicle bones.
Thus, it is seen that the implantable microphone system provided by the invention is directed broadly to systems and methods for detecting relative motion of the functioning middle ear components.
The present invention offers the advantage of an implantable microphone system that uses many of the acoustic properties of the ear that nature provided. That is, because both the outer ear and middle ear components are used, the directional performance for sensing sound is enhanced. Moreover, there may be, for some patients, a natural stapedius response provided by the natural tightening of the tympanic membrane by the stapedius tendon. Further, the location of the device in the middle ear also provides protection from the outside environment as well as a cosmetic enhancement.
The invention may also be characterized as a method of sensing sound using implantable components and generating a microphone signal representative of the sensed sound, which microphone signal is useable by a cochlear implant or other hearing aid device. The method comprises the steps of: (a) implanting a motion sensor in the middle ear, the motion sensor including means for sensing movement of at least one middle ear component; (b) sensing motion of at least one of the moveable middle ear components using the implanted motion sensor; and (c) converting the sensed motion of at least one middle ear component to the microphone signal representative of sensed sound. In one embodiment, the motion sensor comprises a linear-variable-differential transformer (LVDT). In another embodiment, the motion sensor comprises a movable plate differential capacitor (MPDC), or equivalent capacitive element.
It is thus an object of the present invention to provide an implantable microphone that may be used with a cochlear implant or other hearing aid system.
It is a further object of the invention to provide an implantable microphone system that utilizes many of the natural acoustic properties of the ear that nature provided, such as the ability to use the outer ear to collect and direct sound into the ear canal, and the ability to use the functioning middle ear components without interference with the motion of such middle ear components through physical contact therewith.